1. Field of the invention
This invention relates to a semiconductor device, and more specifically to a semiconductor device to be customized with a use of wirings.
2. Description of the Related Art
Recently, there is a remarkable increase in the integration density of semiconductor devices. This increase tends to cause the manufacturing process of the semiconductor device quite complex. Regarding custom products, the complex manufacturing process results not only in a higher manufacturing cost but also a longer delivery time than non-custom products. In an attempt to overcome these drawbacks, many suppliers design basic circuit modules, such as a gate circuit, memory cells, a decoder, and a CPU, which are formed on the semiconductor substrate as a gate array or an ASIC (Application Specific Integration Circuit), and connect these modules to one another in accordance with the customer's order. In this way, the manufacturing cost of the custom product can be lowered and the time required for delivery shortened in comparison with the case where circuit modules are customs-designed for each and every customer, upon receipt of an order.
An example of a custom product is a semiconductor memory device which is programmed by the manufacture himself; for example, a mask ROM. A typical mask ROM comprises a plurality of memory cells each of which is constituted by, for example, an MOS transistor, each MOS transistor having a gate connected so as to receive a reading voltage, a source connected to a reference potential terminal, and a drain selectively connected to a data output line, in accordance with data "1" or "0" in the programming. The data output line is connected to the power source potential terminal via a load circuit. The reading voltage is applied to one of the MOS transistors during the reading operation, so that the MOS transistor is in an ON state. At this time, the data output line is set at a potential equal to that of the reference potential terminal if the drain of the MOS transistor is connected to the data output line, and at a potential equal to that of the power source potential terminal if the drain is open.
Semiconductor memory devices such as that described above are often incorporated in equipment which is in temporary demand. Consequently, the volume of sales of such equipment may well depend on the speed at which the semiconductor memory device in question can be delivered. Therefore, it is necessary that such a semiconductor memory device be able to be delivered within a much shorter period of time than standard custom products.
However, the conventional custom semiconductor memory device requires a relatively large number of manufacturing steps following the reception of an order; i.e., the step of depositing an aluminum layer on the semiconductor structure including memory cells; patterning the aluminum layer to form a wiring pattern including wiring lines for connecting memory cells storing data "1" to the data output line; forming a passivation layer which covers the wiring lines; forming openings in the passivation layer so that part of the wiring pattern can be used as a bonding pad. Because of the number of the steps involved, it is therefore difficult to deliver a conventional semiconductor memory device to the customer within a relatively short period of time.